[Ifeffit] Effect of gradual thickness variation in beam

[Jason Gaudet]

Sample cell

Hi all,

I'm planning on some transmission-mode XAS with smaller than usual sample
tubes.  I'm realizing I might be in danger of creating excessive
non-uniformity in my samples by having the beam size on the same order of
magnitude as the sample tube radius.

For example, let's say I want to measure liquid in a sample tube with a
1000 micron outer diameter, with a beam 500 microns wide and centered on
the sample tube.  If the tube is orthogonal to the ring plane, the entire
vertical portion of the beam will pass through the same length of liquid.
But in the horizontal plane, the center of the beam will pass through 1000
microns of sample while the edges of the beam will pass through 866 microns
of sample, due to the curvature of the sample cell across the horizontal
plane.

Most of what I know about the statistics of thickness effects are about
leakage and pinholes - nonlinearity caused by a few spots having very low
or negligible sample thickness.  But I don't know how significant a "mild"
thickness distribution might be.  If this sort of thickness distribution is
going to be an issue it would be great to know that beforehand and either
go with larger samples, smaller beam size, or more creative orientation.

Thanks,

Jason
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